1. Field of the Invention
The present invention relates to a compressor having a rotation detecting mechanism for detecting an unusual situation.
2. Description of the Related Art
Compressors for compressing gas, especially compressors used in automobile refrigerating systems must be constructed in a hermetic structure. When a compressor fails to rotate due to, for example, frictional overheating or attempting to compress a liquid fluid, it is necessary to rapidly disconnect the compressor from its driving system with a drive source (for example, an engine) to prevent damage to the automobile power train. Especially, in a compressor driven by a driving system which also drives auxiliary devices of an automobile such as a water pump and an alternator, via a magnetic clutch, it is important to turn the magnetic clutch off as soon as an unusual situation occurs so that the auxiliary devices are not affected by the damaged compressor. Accordingly, the compressor conventionally includes a rotation detecting mechanism for detecting an unusual situation of the compressor so as to turn the magnetic clutch off to disconnect the compressor from its driving source when the rotation of the compressor falls below a predetermined value.
For example, Japanese Examined Utility Model Publication (Kokoku) No. 3-6878 discloses a wobble plate type compressor including a rotation detecting mechanism which comprises a magnetic circuit formed such that a magnetic flux leaking from a magnetic clutch is guided to a bearing race of a rotatable member of the compressor via a drive shaft, and a magnetic sensor (a magnetic transducer arranged in the casing of the compressor for cooperation with the rotatable member. A projection is arranged on the periphery of the bearing race so that the projection periodically moves across the magnetic sensor and the latter can periodically detect a change in the magnetic flux with the rotation of the rotatable member.
Japanese Unexamined Utility Model Publication (Kokai) No. 58-73991 discloses a slide vane type compressor including a rotation detecting mechanism which comprises a magnetic circuit formed such that a magnetic flux leaking from a magnetic clutch is guided to an iron core of a sensor via a drive shaft and a rotor (vane), the sensor being arranged in the casing of the compressor so as to face the end face of the rotor (vane) to thus detect a change in a magnetic flux in a manner similar to the previous case.
In the detection of the rotation of the compressor having a hermetic structure, a passage of detected signals, a space for mounting a magnetic sensor, and an output level of detected signals must be taken into consideration. If the sensor and/or its cables extend from the interior to the exterior of the compressor through the casing, a sealing arrangement must be added to a mounting place of the sensor and/or its cables. Also, if the detection of the rotation of the compressor is carried out by using a drive shaft or a pulley cooperating with the drive shaft, it is difficult to find a mounting place for a magnetic sensor in the casing of the compressor. Also, if a sensor is arranged outside the casing of the compressor and the magnetic sensor must detect a change in a magnetic flux which appears within the casing of the compressor, through the wall of the casing of the compressor, an output level of detected signals from the magnetic sensor will be low.
In view of these considerations, the rotation detecting mechanisms of the above described prior arts have an economical disadvantage because a special sealing arrangement must be added. Also, in the first described prior art, it is difficult to form a closed magnetic circuit because the magnetic sensor is arranged at a position remote from the magnetic clutch and the sensitivity of the sensor is low.